Make `distinct()` call before replacing original `qs` reference (#283).
Make `distinct()` call before replacing original `qs` reference (#283).

By overwriting the reference to the original qs/mock it gets hard to assert that
`distinct()` was called in the tests. By calling `distinct()` before replacing
the reference to the original qs/mock the tests can stay the same.

alex/django-filter,alex/django-filter,newfylox/django-filter,z0by/django-filter,thenewguy/django-filter,jpadilla/django-filter,z0by/django-filter,thenewguy/django-filter,jpadilla/django-filter,sbellem/django-filter,newfylox/django-filter,sbellem/django-filter